Fix small race condition with username reservation.

feature/registration/src/main/java/org/signal/registration/screens/addusername/AddUsernameViewModel.kt

@@ -126,6 +126,9 @@ class AddUsernameViewModel(
    * Non-digits are dropped as they're typed, since a discriminator can only ever be digits.
    *
    * Emptying the field hands control back to the service once the field loses focus (see [applyDiscriminatorFocusLost]).
+   * Until then we hold onto a service-assigned reservation, since that pairing is still the one the entry is showing.
+   * A user-typed reservation, on the other hand, is dropped as soon as its digits are erased -- the user asked for that
+   * exact number and no longer wants it, so there is nothing left to submit until the field settles.
    */
   private fun applyDiscriminatorChanged(state: AddUsernameState, discriminator: String, stateEmitter: (AddUsernameState) -> Unit) {
     val digitsOnly = discriminator.filter { it in '0'..'9' }
@@ -141,7 +144,7 @@ class AddUsernameViewModel(
       discriminator = digitsOnly,
       isDiscriminatorUserSet = isUserSet,
       validationError = null,
-      reservation = if (isUserSet) null else state.reservation,
+      reservation = state.reservation.takeIf { !isUserSet && !state.isDiscriminatorUserSet },
       isReserving = false
     )
 

feature/registration/src/test/java/org/signal/registration/screens/addusername/AddUsernameViewModelTest.kt

@@ -205,6 +205,42 @@ class AddUsernameViewModelTest {
     assertThat(viewModel.state.value.isSubmittable).isTrue()
   }
 
+  @Test
+  fun `emptying a user-typed discriminator drops its reservation`() = runTest(testDispatcher) {
+    coEvery { mockRepository.reserveUsername("maya") } returns RequestResult.Success(Username("maya.45"))
+    coEvery { mockRepository.reserveUsername("maya", "77") } returns RequestResult.Success(Username("maya.77"))
+
+    viewModel.onEvent(AddUsernameScreenEvents.UsernameChanged("maya"))
+    advanceUntilIdle()
+    viewModel.onEvent(AddUsernameScreenEvents.DiscriminatorChanged("77"))
+    advanceUntilIdle()
+    viewModel.onEvent(AddUsernameScreenEvents.DiscriminatorChanged(""))
+    advanceUntilIdle()
+
+    assertThat(viewModel.state.value.discriminator).isEmpty()
+    assertThat(viewModel.state.value.isDiscriminatorUserSet).isFalse()
+    assertThat(viewModel.state.value.reservation).isNull()
+    assertThat(viewModel.state.value.isSubmittable).isFalse()
+  }
+
+  @Test
+  fun `NextClicked cannot confirm a discriminator the user just erased`() = runTest(testDispatcher) {
+    coEvery { mockRepository.reserveUsername("maya") } returns RequestResult.Success(Username("maya.45"))
+    coEvery { mockRepository.reserveUsername("maya", "77") } returns RequestResult.Success(Username("maya.77"))
+
+    viewModel.onEvent(AddUsernameScreenEvents.UsernameChanged("maya"))
+    advanceUntilIdle()
+    viewModel.onEvent(AddUsernameScreenEvents.DiscriminatorChanged("77"))
+    advanceUntilIdle()
+    viewModel.onEvent(AddUsernameScreenEvents.DiscriminatorChanged(""))
+    advanceUntilIdle()
+    viewModel.onEvent(AddUsernameScreenEvents.NextClicked)
+    advanceUntilIdle()
+
+    coVerify(exactly = 0) { mockRepository.confirmUsername(any()) }
+    assertThat(parentEvents).isEmpty()
+  }
+
   @Test
   fun `losing focus with an empty discriminator hands it back to the service`() = runTest(testDispatcher) {
     coEvery { mockRepository.reserveUsername("maya") } returns RequestResult.Success(Username("maya.45")) andThen RequestResult.Success(Username("maya.99"))